Because of numerous advantageous functional properties, for example, the high energy densities and industrially relevant current densities during charging and also discharging procedures, electrochemical storage devices, including in particular metal-metal halogenide and sodium-sulfur storage batteries, are increasingly also being used in large-scale industrial applications. To monitor the service life of such an electrochemical storage device and to lengthen it by way of maintenance measures, it is advantageous to detect the charge or discharge state of the electrochemical storage device safely and reliably.
In the cell design known from the prior art, for example, of the electrochemical storage device based on the technology of the sodium-nickel chloride cell, the anode or negative electrode has, at operating temperatures, liquid metal, especially sodium, as the electrochemical storage material. The fill level of the liquid metal changes as a result of the charging or discharging procedures in this storage device. With increasing charge of this storage device based on the technology of the sodium-nickel chloride cell, the sodium fill level rises in an anode chamber interconnected with the anode of the storage device, whereby changes of the electrical and also electrochemical parameters result (for example, a change of the charge curve). A comparable change also takes place upon increasing discharge of the storage device, wherein the sodium fill level then sinks accordingly. The difference of the fill levels can be up to 10 cm or more between a fully charged storage device, which is based on the technology of the sodium-nickel chloride cell, and a completely discharged storage device.
It has thus proven to be technically advantageous to predict the state of such an electrochemical storage device, in particular the charge or discharge state of such an electrochemical storage device, with sufficient reliability and in a controlled manner. A method for direct measurement of the fill level in a sodium-nickel chloride storage device is described, for example, in the patent application US 2011/0050235. It is proposed therein that, by means of two metal feelers, which are introduced from above into a storage device, the electrical short-circuit be detected when both feelers are short-circuited by contact with the liquid metal. However, the complex mechanical integration of these measuring feelers in the storage device is disadvantageous in such a solution, which in particular requires difficult sealing measures. In addition, retrofitting already manufactured storage devices is hardly possible. Moreover, in such a solution, only the binary information can be ascertained as to whether the present fill level is above or below the position of the contact pins; intermediate values of the fill level cannot be determined directly according to the prior art.
A further method for measuring the fill level in a high-temperature battery is described in DE 10 2008 043 193 A1. According to this teaching, the fill level can be ascertained by means of a metallic resistance strip arranged in the battery.
A further alternative solution is described in US 2011/0236749 A1, in which the fill level is to be ascertained by means of communicating pipes. This solution still has the disadvantage of the high technical expenditure, however, which is required for a state detection in the electrochemical storage device. In addition, the solution again does not permit retrofitting of existing cells using a technology which could enable the state detection.
A further technical solution is specified in DE 42 29 735 A1, according to which the state of an acid chamber, in particular of a lead acid battery, can be ascertained by means of an ultrasonic wave generator and an ultrasonic wave receiving device. According to the device described therein, ultrasonic waves are emitted into the interior of the battery, to propagate in the acid chamber. Reflected components are recorded by the ultrasonic wave receiving device and analyzed, whereby inferences can be obtained about the acid state. The ultrasonic wave generator and the ultrasonic wave receiving device are placed for this purpose on an opening of the battery, which also forms the emission channel or detection channel.
However, it has proven to be disadvantageous for such a state detection that an open connection has to exist between acid chamber and ultrasonic wave generator or ultrasonic wave receiving device. For this purpose, it is not only necessary to firstly open the battery chamber, in order to attach the ultrasonic wave generator and the ultrasonic wave receiving device, but rather also to seal them in relation to the surroundings. In particular in the case of reactive substances in the battery chamber, an undesired escape of battery liquid can thus easily occur.
In addition, such a device is not suitable for use in conjunction with high-temperature batteries, in which the formation of leaks is a concern because of the thermal variations during operation. Furthermore, such a device has proven to be unfavorable during maintenance work on the ultrasonic wave generator or the ultrasonic wave receiving device, since they always have to be removed from the battery housing, and thus expose the battery chamber.
It has thus proven to be technically desirable to avoid these disadvantages known from the prior art. In addition, it is technically desirable to execute such a state detection independently of the ambient medium into which the electrochemical storage device is introduced, with sufficient reliability and accuracy. It is to be taken into consideration in this case that electrochemical storage devices, for example, the sodium-nickel chloride cell or the sodium-sulfur cell, can only be operated at high temperatures from time to time, for example, of greater than 250° C., in particular even of greater than 350° C. Furthermore, it is technically desirable to propose a technology which is able to detect rapidly and is also simultaneously usable for numerous individual electrochemical storage devices, without a space-saving interconnection of these storage devices coming into consideration. Furthermore, it is desirable, in particular when interconnecting many electrochemical storage devices to form larger modules, to query individual states, in particular the charge or discharge state, of a storage device continuously and in a targeted manner. Moreover, such a technology is to be robust and cost-effective. The resulting maintenance expenditure for such a technology is also to be low or the technology is to be maintenance-free.